Translocation and accumulation of translocate in the sugar beet petiole

Accumulation of translocate during steady-state labeling of photosynthate was measured in the source leaf petioles of sugar beet (Beta vulgaris L. monogerm hybrid). During an 8-hr period, 2.7% of the translocate or 0.38 μg carbon/min was accumulated per cm petiole. Material was stored mainly as sucrose and as compounds insoluble in 80% ethanol. The minimum peak velocity of translocation approached an average of 54 cm/hr as the specific activity of the ¹⁴CO₂ pulse was progressively increased. The ratio of cross sectional area required for translocation to actual sieve tube area in the petiole was 1.2. A regression analysis of translocation rate versus sieve tube cross sectional area yielded a coefficient of 0.76. The specific mass transfer rate in the petiole was 1.4 g/hr cm² phloem or 4.8 g/hr cm² sieve tube. Histoautoradiographic studies indicated that translocation occurs through the area of phloem occupied by sieve tubes and companion cells while storage occurs in these cells plus cambium and phloem parenchyma cells. The ability of the petiole to act as a sink for translocate is consistent with the concept that storage along path tissue serves to buffer sucrose concentration in the translocate during periods of fluctuating assimilation.     

Structure and development of sieve cells in the secondary phloem of Larix decidua Mill. as related to function

Summary Only one or two layers of sieve cells of the previous year's phloem in lateral branches of Larix decidua persist as fully mature cells. Immature sieve cells or cambial derivatives that have not completed differentiation may also over-winter. Periclinal cell divisions of the vascular cambium were first observed by mid-April. During the short period of greatest cambium activity (mid-April to mid-May), the early phloem is laid down. Late phloem is formed over a much longer period, from mid-May to late September. Microautoradiography revealed that only mature sieve cells of the early phloem are involved in translocation of ¹⁴C assimilates in June. The fine structure of actively translocating sieve cells is described. The impact of structure on long-distance transport of assimilates is discussed.     

Seasonal development of phloem in scots pine stems

The formation of phloem was studied for two years in stems of 50 to 60 year old trees of Scots pine (Pinus sylvestris L.) growing in nature. The development of phloem of the current year begins 10 to 20 days before the xylem formation and is completed with the termination of shoot growth in the end of June. Observations over the seasonal activity of cambium producing sieve cells of phloem and duration of their differentiation as compared to the xylem derivatives of cambium have shown that the maxima of formation of phloem and xylem cells could coincide or not coincide by season, while the activities of their differentiation were always in antiphase. The sieve cells of early phloem were separated from those of late phloem by a layer of tannin-containing cells, which are formed simultaneously with the formation of late xylem cells by the cambium. Seasonal dynamics of accumulation of starch grain in structural elements of the phloem is related to the xylem development. The content of metabolites in differentiating and mature phloem elements, in the cambium zone, and in the xylem cells growing in the radial direction depended on cell specificity, stage of their development, and type of forming wood, early or late, which differ in the cell wall parameters and, hence, requirement of assimilates. Significant differences were described between the content of low molecular weigh carbohydrates, amino acids, organic acids, and phenol compounds using two methods of calculation: per dry weight and per cell.     

Conductance of needle and twig axis phloem of damaged and intact Norway spruce (Picea abies (L.) Karst.) as investigated by application of14C in situ

Summary Phloem conductance of¹⁴C-labelled assimilates was investigated in natural stands of Norway spruce showing substantial damage from needle yellowing and needle loss disease. Terminal current-year shoots of a branch were allowed to fix¹⁴CO₂ (300–600 ppm in air) and carbon dioxide net uptake was monitored with a gas analyser. The difference between¹⁴C-uptake and the amount of radiocarbon determined in the photosynthesizing needles was interpreted to reflect assimilate export from the needles to the axis of the tree. Compared with an undamaged control tree,¹⁴C-export from the assimilating needles was not impaired in the yellowing tree and only slightly reduced in the tree showing needle loss. Incorporation of¹⁴C into starch increased significantly during autumn particularly in the tree showing needle loss. Import of radiocarbon from the¹⁴C-labelled phloem sap in twig axes and needles older than 1 year was used as a measure of phloem conductivity of older sections of a branch which showed considerable damage. Carbon uptake by these older plant parts was more pronounced than in undamaged twigs. In the case of older needles enhancement of¹⁴C-incorporation suggested an increased sink strength, while the same phenomenon in the twig axes was interpreted as a consequence of partially impaired conductivity of individual sieve elements resulting in an inhomogeneous velocity of phloem transport. The hypothesis is put forward that curtailed viability of the sieve cells is responsible for a delay of transport, which is compensated for by an augmented production of phloem elements from the cambium.     

Stem anatomy and development of inter- and intraxylary phloem in Leptadenia pyrotechnica (Forssk.) Decne. (Asclepiadaceae)

Modification of external morphology and internal structure of plants is a key feature of their successful survival in extreme habitats. They adapt to arid habitats not only by modifying their leaves, but also show several modifications in their conducting system. Therefore, the present study is aimed to investigate the pattern of secondary growth in Leptadenia pyrotechnica (Forssk.) Decne., (Asclepiadaceae), one such species growing in Kachchh district, an arid region of Gujarat State. A single ring of vascular cambium, responsible for radial growth, divided bidirectionally and formed the secondary xylem centripetally and the phloem centrifugally. After a short period of secondary xylem differentiation, small arcs of cambium began to form secondary phloem centripetally instead of secondary xylem. After a short duration of such secondary phloem formation, these segments of cambium resumed their normal function to produce secondary xylem internally. Thus, the phloem strands became embedded within the secondary xylem and formed interxylary phloem islands. Such a recurrent behavior of the vascular cambium resulted in the formation of several patches of interxylary phloem islands. In thick stems the earlier formed non-conducting interxylary phloem showed heavy accumulation of callose on the sieve plates followed by their crushing in response to the addition of new sieve elements. Development of intraxylary phloem is also observed from the cells situated on the pith margin. As secondary growth progresses further, small arcs of internal cambium get initiated between the protoxylem and intraxylary phloem. In the secondary xylem, some of the vessels are exceptionally thick-walled, which may be associated with dry habitats in order to protect the vessel from collapsing during the dryer part of the year. The inter- and intraxylary phloem may also be an adaptive feature to prevent the sieve elements to become non-conducting during summer when the temperature is much higher.     

Wood Anatomy and the Development of Interxylary Phloem of Ipomoea hederifolia Linn. (Convolvulaceae)

In Ipomoea hederifolia Linn., stems increase in thickness by forming successive rings of cambia. With the increase in stem diameter, the first ring of cambium also gives rise to thin-walled parenchymatous islands along with thick-walled xylem derivatives to its inner side. The size of these islands increases (both radially and tangentially) gradually with the increase in stem diameter. In pencil-thick stems, that is, before the differentiation of a second ring of cambium, some of the parenchyma cells within these islands differentiate into interxylary phloem. Although all successive cambia forms secondary phloem continuously, simultaneous development of interxylary phloem was observed in the innermost successive ring of xylem. In the mature stems, thick-walled parenchyma cells formed at the beginning of secondary growth underwent dedifferentiation and led to the formation of phloem derivatives. Structurally, sieve tube elements showed both simple sieve plates on transverse to slightly oblique end walls and compound sieve plates on the oblique end walls with poorly developed lateral sieve areas. Isolated or groups of two to three sieve elements were noticed in the rays of secondary phloem. They possessed simple sieve plates with distinct companion cells at their corners. The length of these elements was more or less similar to that of ray parenchyma cells but their diameter was slightly less. Similarly, in the secondary xylem, perforated ray cells were noticed in the innermost xylem ring. They were larger than the adjacent ray cells and possessed oval to circular simple perforation plates. The structures of interxylary phloem, perforated ray cells, and ray sieve elements are described in detail.     

THE SEASONAL CYCLE OF PHLOEM DEVELOPMENT IN JUNIPERUS CALIFORNICA

In Juniperus californica, all sieve cells of the previous season's phloem growth increment overwinter in a mature state. Initiation of cambial activity begins in early March and, by the end of March, the oldest sieve cells that overwintered lose their contents and die. By mid-April, even the youngest sieve cells of the previous season's growth increment have lost their contents. The period of greatest cambial activity begins in the last half of April and continues through May. With the slowing of cambial activity in June, callose begins to collect on the sieve areas of the first-formed sieve cells of the new increment. By July, the cambium and phloem are in a dormant state. Initiation of phloem production precedes that of the xylem by about 1 month. Production of new xylem and phloem ceases simultaneously in July.     

Phosphorus compounds in translocating Phloem

Phosphate-³²P was introduced into a turnip leaf, and 3 hr later, the vascular bundles were stripped from the petiole and their phosphate ester pattern was studied. The pattern did not alter along their length and was like that of other tissues. Pumpkin leaves were painted with phosphate-³²P; and later, the petioles were cut, the sieve tube exudates were collected and their phosphate ester patterns were studied. Exudates collected after 10 min had a high proportion of their ³²P present in P_i and nucleoside triphosphates, while exudates collected after long translocation times (4-22 hr) had a lower proportion in these, and a higher proportion in hexose monophosphates and UDP glucose. In general, the ester patterns were like those of other tissues. The results indicate that sieve tubes are metabolically active, and that P_i is the primary form in which phosphorus moves in the phloem.     

SEASONAL DEVELOPMENT OF THE SECONDARY PHLOEM IN PINUS

Functional sieve cells are present at all times in the secondary phloem of Pinus banksiana Lamb., P. resinosa Ait., and P. strobus L. With regard to a given year's growth increment, all but the last-formed sieve cells (2-4 layers) cease functioning the same season they are derived from the cambium. The former overwinter and remain functional until new sieve cells differentiate in spring. Toward the end of March undifferentiated cells in the outer margin of the cambial zone begin to differentiate into sieve cells. About a week later, cambial activity (cell division) commences. All early phloem is produced by early May before new xylem differentiation begins. Most sieve cells are differentiated by late August, but a few not until late September. Cessation of function begins in late May or June with formation of definitive callose on sieve areas of the sieve cells which overwintered and continues slowly to sieve cells of the current season's early phloem. By mid-December all but the last-formed sieve cells (i.e., those which will overwinter in a functional state) are devoid of contents. Phloem differentiation precedes xylem differentiation by approximately 1 1/2 months. Xylem and phloem production cease more or less simultaneously in August, xylem and phloem differentiation in September.     

Seasonal dynamics of phloem and xylem formation in silver fir and Norway spruce as affected by drought

The dynamics of phloem growth ring formation in silver fir (Abies alba Mill.) and Norway spruce (Picea abies Karst.) at different sites in Slovenia during the droughty growing season of 2003 was studied. We also determined the timing of cambial activity, xylem and phloem formation, and counted the number of cells in the completed phloem and xylem growth rings. Light microscopy of cross-sections revealed that cambial activity started on the phloem and xylem side simultaneously at all three plots. However, prior to this, 1–2 layers of phloem derivatives near the cambium were differentiated without previous divisions. The structure of the early phloem was similar in silver fir and Norway spruce. Differences in the number of late phloem cells were found among sites. Phloem growth rings were the widest in Norway spruce growing at the lowland site. In all investigated trees, the cambium produced 5–12 times more xylem cells than phloem ones. In addition, the variability in the number of cells in the 2003 growth ring around the stem circumference of the same tree and among different trees was higher on the xylem side than on the phloem side. Phloem formation is presumably less dependent on environmental factors but is more internally driven than xylem formation.     

Changes in the amino acid composition and protein modification in phloem sap of rice

Pure phloem sap was collected from insects feeding on rice (Oryza sativa L.) leaves by a laser technique similar to the aphid stylet technique. Rapid circulation of nitrogen in the sieve tubes was demonstrated directly using ¹⁵N as a tracer. Application to the roots of the metabolic inhibitors of amino acids, aminooxyacetate and methioninesulfoximine, changed the amino acid composition in the sieve tubes. Feeding methionine to leaf tips resulted in its bulk transfer into the sieve tubes. In vitro experiments confirmed the existence of protein kinases in the pure rice phloem sap. The phosphorylation status of the sieve tube sap proteins was affected by the light regime. The possibility that changes in chemical composition or protein modification such as phosphorylation in the sieve tubes might affect plant growth are discussed.Analysis of pure phloem sap collected from rice plants by insect laser technique has shown dynamic changes in the chemical composition and the quality of proteins in the sap.     

Enhancement of Phloem Exudation from Fraxinus uhdei Wenz. (Evergreen Ash) using Ethylenediaminetetraacetic Acid

Ethylenediaminetetraacetic acid (EDTA) enhanced the exudation of ¹⁴C-labeled assimilates from excised leaflets and whole plant specimens of Fraxinus uhdei Wenz. A 2 millimolar EDTA concentration was found to be most effective in promoting exudation from excised leaflets, while 10 millimolar EDTA was most effective in whole plants experiments. Exudation rate reached a maximum after 24 hours in both experiments. The continuous presence of EDTA throughout the treatment period was required for maximum exudation from excised leaflets. Stachyose, raffinose, verbascose, and sucrose were the principal compounds found to occur in exudate samples. These compounds are typically transported in sieve elements of various Fraxinus species suggesting the exudate was of phloem origin. Electron microscope studies of petiolule sieve plate pores from excised leaflets showed substantially less callose appearing after treatment with EDTA than after H₂O treatment. It is suggested that EDTA enhances phloem exudation by inhibiting or reducing callose formation in sieve plate pores. The exudation enhancement technique described for whole plant specimens is suggested as a useful means of collecting phloem sap and studying translocation in woody plants.     

Formation of successive cambia and stem anatomy of Sesuvium sesuvioides (Aizoaceae)

Mature stems of Sesuvium sesuvioides (Fenzl) Verdc. were found to be composed of successive rings of xylem alternating with phloem. Repeated periclinal divisions in the parenchyma outside the primary phloem gave rise to conjunctive tissue and the lateral meristem that differentiate into the vascular cambium on its inner side. After the formation of the vascular cambium, the lateral meristem external to it became indistinct as long as the cambium was functional. As the cambium ceased to divide, the lateral meristem again became apparent prior to the initiation of the next cambial ring. The cambium was exclusively composed of fusiform cambial cells with no rays. In the young saplings, the number of cambial cylinders in the axis varied from the apex to the base, indicating formation of several rings within the year. In each successive ring of the lateral meristem, small segments differentiated into the vascular cambium and gave rise to vessels, axial parenchyma, fibres and fibriform vessels towards the inside, and secondary phloem on the outer side. In the old stems, non‐functional phloem of the innermost rings was replaced by a new set of sieve tube elements formed by periclinal divisions in the cambial segments associated with the non‐functional phloem. In some places the cambial segments completely differentiate into derivatives leaving no cambial cells between the xylem and phloem. © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society, 2008, 158 , 548–555.     

Distribution of an Indoleacetic Acid-oxidase-inhibitor in the Storage Root of Daucus carota

Indoleacetic acid (IAA)-oxidase from both secondary phloem and xylem was dependent on 2,4-dichlorophenol for activity, and was enhanced by addition of Mn²⁺. The pH optimum was 6.0 from both tissues. IAA-oxidase and its inhibitors were distributed differently in the secondary phloem and secondary xylem of carrot root. In the phloem a high IAA-oxidase activity was distributed uniformly along the radius but in the xylem a somewhat lower concentration decreased from the cambium. IAA-oxidase inhibitor in the phloem increased exponentially from a very low concentration near the cambium, whereas in the xylem an appreciable concentration was present near the cambium, decreasing linearly with distance from the cambium. Longitudinal gradients in the xylem parallel studies by other workers with the greatest IAA-destroying capacity present in older tissues. In the xylem inhibitor decreased and IAA-oxidase increased from the root apex. In the phloem IAA-oxidase was uniform, whereas the inhibitor increased in older tissue.

The IAA-oxidase inhibitors in phloem and xylem may be different. In the xylem the IAA-oxidase inhibitor may be a lignin precursor present in young cells which disappears as lignification proceeds. In the phloem IAA-oxidase reacting with endogenous IAA appears to form a physiologically active product.

     

Zur Lokalisierung der funktionstüchtigen Siebzellen im sekundären Phloem von Metasequoia glyptostroboides

Zusammenfassung Der Transport von ¹⁴C-markierten Assimilaten und der Verlauf der Stechborsten von Aphiden im Phloem eines zweijährigen Zweiges von Metasequoia glyptostroboides wurden mit dem Ziel untersucht, den Bereich der funktionstüchtigen Siebzellen genau zu lokalisieren. Mit Hilfe einer neuen Methode der Gefrierschnitt-Autoradiographie kann gezeigt werden, daß sich die Radioaktivität stets im Bereich der jüngsten Phloemelemente zwischen Cambium und erstem Bastfaserring befindet. Auf Metasequoia parasitierende Rindenläuse stechen gleichfalls stets diejenigen jungen Siebzellen an, die in unmittelbarer Nachbarschaft zum ersten Bastfaserring liegen.

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Localization of the functioning sieve cells in secondary phloem tissue of Metasequoia glyptostroboides

Summary Translocation of ¹⁴C-labeled assimilates and the path of aphid stylets in the phloem tissue have been examined in order to localize the region of functioning sieve cells in a two-year-old branch of Metasequoia glyptostroboides in October. A new method of microautoradiography using cryostat sections is described. Radioactivity was found only in the region of the youngest phloem cells adjacent to the cambium. Aphids feeding on Metasequoia pierce those sieve cells associated with the first band of bast fibers.

     

Stem anatomy of the dwarf subshrub Cressa cretica L. (Convolvulaceae)

Stem anatomy and development of medullary phloem are studied in the dwarf subshrub Cressa cretica L. (Convolvulaceae). The family Convolvulaceae is dominated by vines or woody climbers, which are characterized by the presence of successive cambia, medullary- and included phloem, internal cambium and presence of fibriform vessels. The main stems of the not winding C. cretica shows presence of medullary (internal) phloem, internal cambium and fibriform vessels, whereas successive cambia and included phloem are lacking. However, presence of fibriform vessels is an unique feature which so far has been reported only in climbing members of the family. Medullary phloem develops from peri-medullary cells after the initiation of secondary growth and completely occupies the pith region in fully grown mature plants. In young stems, the cortex is wide and formed of radial files of tightly packed small and large cells without intercellular air spaces. In thick stems, cortical cells become compressed due to the pressure developed by the radial expansion of secondary xylem, a feature actually common to halophytes. The stem diameter increases by the activity of a single ring of vascular cambium. The secondary xylem is composed of vessels (both wide and fibriform), fibres, axial parenchyma cells and uni-seriate rays. The secondary phloem consists of sieve elements, companion cells, axial and ray parenchyma cells. In consequence, Cressa shares anatomical characteristics of both climbing and non-climbing members. The structure of the secondary xylem is correlated with the habit and comparable with that of other climbing members of Convolvulaceae.     

RESPIRATORY METABOLISM IN THE PHLOEM, XYLEM AND CAMBIUM OF CARROT ROOT

Comparisons of respiratory metabolism among the phloem, cambiumand xylem of mature carrot root were carried out and the followingresults were obtained. 1) The activity of O₂ uptake on a dry weight basis in the cambiumwas higher than that in the xylem. The phloem showed the lowestactivity. 2) RQ was close to unity in all the three tissues.3) The inhibition rate of O₂ uptake by malonate was considerablyhigh in the three tissues. The highest inhibition was observedin the cambium. 4) In the phloem and xylem the malonic inhibitionrate of ¹⁴CO₂ release from G-U-¹⁴C was not so high as comparedwith that of O₂ uptake. In the cambium, however, those of O₂uptake and ¹⁴CO₂ output were comparable. 5) Malonate treatmentdid not cause any significant change in RQ. 6) The C₆/C₁ ratiowas highest in the cambium and lowest in the phloem. From these it is concluded that the participation of the TCAcycle in respiration is great in all the three tissues of carrotroot, and among the three the cambium has the highest activityof the cycle. As for the PP pathway, the xylem and phloem havea higher activity than the cambium. (Received April 11, 1967; )     

THE TIME COURSE OF SIEVE TUBE AND VESSEL REGENERATION AND THEIR RELATION TO PHLOEM ANASTOMOSES IN MATURE INTERNODES OF COLEUS

Quantitative counts of regenerative sieve tubes and vessels were made in a large number of samples of mature internode #5 of C. blumei, with concomitant study of the fine details of vascular regeneration and the occurrence of the normally developing phloem anastomoses. Such anastomoses were found in many of the plants, but their average number in the small regenerating area was low (viz., 0.9 ± 0.2). With the phloem anastomoses excluded from the counts, the time course of regeneration was clear cut—no strands completed their regeneration around the wound until three days after wounding. More regenerative sieve tubes completed their differentiation under all conditions than did regenerative vessels. The number of sieve tubes and vessels regenerated by four days was closely related to the number of preexisting bundles of that type of vascular cell that had been severed by the transverse wound. The ratio of bundles severed by the wound in the phloem to those in the xylem was 2.14, and the ratio of the regenerative sieve tubes to the regenerative vessels was 2.24. For both tracheary and sieve tube cells the initial regeneration was strongly polar (mostly above the wound), as expected from earlier IAA transport data. The path of tracheary regeneration was obviously related to that of the sieve tubes on the other side of the cambium.     

Periodicity of Cambium Activity and Seasonal Changes of the Secondary Phloem in Two Species of Dalbergia

Periodicity of cambium activity, seasonal changes of the secondary phloem and longevity of sieve tube in main trunk of Dalbergia balansae Prain and in the twig of D. szemaoensis Prain were observed. The results are as follows: 1. All cambia fall under the category of storied type. 2. In D. balansae cambial activity begins in late April and ends in early November. Phloem differentiation is completed by early November. Xylem differentiation ceases in December. In D. szemaoensis cambial activity continues from mid-April to late October. Phloem and xylem differentiation ceases by late November. 3. The width of functional phloem zone is maximal (400—600 μm) in autumn and minimal (200—370 μm) in February to April. In overwintering, functional sieve tube elements contain P-protein, and the pores of sieve plate are open. It could be one of the reasons that these two species are promising host trees of Kerria yunnanensis during winter. 4. The longevity of sieve tubes in D. balansae and D. szemaoensis last 8—12 months and 9—11 months respectively. 5. During dormancy of cambium, the parenchyma cells of the secondary phloem contain large quantities of starch grains and calcium oxalate crystals, which decrease as cambium becomes active and remain little or even non visualized in summer.